This invention relates to cathode ray tubes and more particularly to a cathode ray tube adapted for protection of inner electrodes and external electric circuits such as video circuit and tuner circuit against spark energy generated within the tube and prevention of the television set from erroneous operation and trouble.
In recent years, earnest demand for improved properties of television sets such as increased image brightness has resulted in widespread practice of setting the anode voltage of cathode ray tubes to a high range of 20 to 30 KV. Consequently, the interior of such a cathode ray tube frequently undergoes various forms of electric discharge such as, for example, surface creepage occurring on the bead glass holding a plurality of electrodes fast in position and sparks due to accumulation of foreign matters. Such electric discharge inherently entails release of huge energy. The discharged electric energy does harm to the oxide substance, which is applied as the source for emission of thermo-electrons to the surface of the heater electrode and weak in structure as compared with the other electrodes, as well as to various external electronic circuits such as the video circuits and tuner circuits. Thus, the frequent electric discharge forms the cause for erroneous operation and even failure of the television set.
To overcome such disadvantages, there has been recently introduced a so-called soft-flash tube wherein a portion of the conductive film applied to the inner wall surface of the funnel envelope is made to offer a high resistance enough to hold down the peak current during electric discharge.
FIG. 1 illustrates, in fragmentary sectional form, a typical soft-flash tube as mentioned above. In the figure, an envelope of the tube consists of a panel 1, a funnel 2a, and a neck 2b. Reference numeral 3 designates a shadow mask disposed inside the panel 1, 4 an inner graphite film of a low resistance deposited on the inner wall surface of the funnel 2a, 5 a high-resistance graphite film of TiO.sub.2 family deposited on the inner wall surface of the funnel 2a, 6 an inner graphite film of a low resistance deposited on the inner wall surface of the neck 2b, 7 an anode voltage supply terminal in the form of an anode button buried in the funnel 2a and adapted to feed a high voltage from the outside to the inner graphite film 4, 8 an electron gun assembly, 9 a bulb spacer contact having a contact piece thereof held in contact with the inner graphite film 6 to form part of the electron gun assembly 8 and also serving to receive and supply the high voltage, 10 a getter carried by the free end of a getter support spring 11 extending from the electron gun assembly, and 12 an outer electroconductive film deposited on the outer surface of the funnel 2a.
In the cathode ray tube constructed as described above, the high anode voltage is supplied from the anode button 7 via the high-resistance graphite film 5 and the bulb spacer contact 9 to the electron gun assembly 8. In this case, since the high-resistance graphite film 5 intervening between the anode button 7 and the bulb spacer contact 9 offers a high resistance of the order of several hundred K.OMEGA., the electric current which flows when a spark occurs between the electrodes of the electron gun assembly 8 and between the electron gun assembly 8 and the inner graphite film 6 of the neck can be lowered to a level of one-several tenth of the level usual with the conventional type. Further, spark current due to discharge current from a capacitor formed by the inner graphite film 4 together with the high-resistance graphite film 5 and the outer conductive film 12 on the funnel 2a is similarly lowered.
Incidentally, the cathode ray tube of the aforementioned construction is generally manufactured by a process as illustrated in FIG. 2. Prior to aging under conditions for the tube in operation, the cathode ray tube is subjected to so-called "spot knocking" in which the foregin particles and metal burrs adhering to the electrodes of the electron gun assembly 8 are burnt off or fused by impulse and heat of high-voltage spark and vaporized for removal. This spot knocking is effected by feeding a high voltage from the anode button 7 thereby forcibly setting up sparks between the high-resistance graphite film 5 and the electron gun assembly 8 and between the electrodes of the electron gun assembly 8.
However, in the cathode ray tube which has the high-resistance graphite film 5 on the inner wall surface of the funnel 2a for the purpose of lowering spark current in operation as described above, the magnitude of spark current for spot knocking is lowered and consequently the effect of the spot knocking manifested in the removal of foreign particles and metal burrs is notably impaired. Thus, there ensues a disadvantage that the withstand voltage characteristic of the tube is seriously degraded.